const arr1 = [1, 3, 8, 9, 17, 18];
const arr2 = [1, 8, 5, 12, 18, 19, 100, 120];
let uncommon = arr1.filter(function(obj) { return arr2.indexOf(obj) == -1; });
uncommon += "," + arr2.filter(function(obj) { return arr1.indexOf(obj) == -1; });
console.log(uncommon);
/*
run:
3,9,17,5,12,19,100,120
*/